1. Field of the Invention
This invention pertains in general to safety apparatus for movable platforms and the like and in particular to a safety clamp device adapted to be releasably mounted on an upright safety cable and affixed to a movable platform supported by a supporting hoist line whereby a supporting hoist line failure results in the load supported thereby being transferred to the safety cable. The invention also pertains to a particularly advantageous arrangement of such a safety clamp device, a movable load such as a platform, a motor drive unit and hoist and safety lines.
2. Description of the Prior Art
U.S. Pat. No. 3,386,530 issued June 4, 1968 to H. L. Thompson teaches a safety clamp device adapted to be slidably mounted on an upright safety cable and connected to a supporting hoist line which is attached to and interchangeably raises and lowers a movable platform. That safety device comprises a body member having an upright safety cable receiving cradle and a pair of spaced apart sidewalls extending outwardly therefrom. Each sidewall has an engaging edge of a predetermined length opposite the cradle. A lever arm sandwiched between the sidewalls is pivotally mounted on the body member for pivotal movement about an axis transverse to the direction of travel of the body member relative to the safety cable. The lever arm member is shiftable between a safety cable engaging position and a safety cable release position. The lever arm includes a safety cable engaging portion adapted to engage the safety cable in the safety cable engaging position. Elongate bar members are pivotally attached to the lever arm at the end opposite the safety cable engaging portion. A hoist line is attached to the upper end of the bar members and the safety platform is attached to the lower end. In the normal release position, the elongate bar members are positioned substantially upright and confront the parallel engaging edges of the sidewalls whereby the cable engaging portion of the lever arm is urged to the release position. In the event of hoist line failure, the elongate arms will shift downward thereby urging the lever arm downward and the cable engaging portion will engage the safety cable thereby clamping the safety cable against the cable receiving cradle of the body member and preventing downward movement of the safety device on the safety cable and arresting downward movement of the platform.
One disadvantage of a safety clamp device as described above is that when the movable platform is being raised or lowered, the body member may pivot and not slide parallel to the safety cable, i.e., it may on occasion cock in a nonparallel position, and therefore will not slide freely along the safety cable. Also under such conditions, wear of the safety cable or of the safety device may occur.
In a prior improvement over the safety clamp device of U.S. Pat. No. 3,386,530, an "L" shaped member or modified bellcrank has one arm pivotally connected to the hoist line, and the apex between the two arms is pivotally connected to the elongate member and the other arm is sandwiched between the sidewalls of the body member. The other arm is allowed to slide between the sidewalls and is adapted such that its lower edge may ride on a stop pin carried by the two sidewalls.
This device also has the disadvantage that it can pivot thereby causing drag and wear. It has a further disadvantage in that the free arm of the bellcrank can improperly wedge under the stop pin.
Although the two aforementioned safety clamp devices are intended for use with movable platforms, neither is particularly well adapted for use with movable platforms of the type having a hoist motor or traction drive unit mounted on the movable platform. Previously, a workman would rely on a lifeline which was fastened to his belt at one end and tied off to the building at its other end.
Safety clamp devices are also shown or described in U.S. Pat. Nos. 3,586,125 and 3,302,750.
Other safety clamp devices are taught or shown in the following U.S. Pat. Nos.: 3,006,431; 3,179,994; 3,317,971; 3,598,200; 3,852,943; Des 180,093 and Des 199,198. However, none of the safety clamp devices shown in these patents are adapted for use with a movable platform.
It is one object of the invention to provide an arrangement whereby a safety clamp device may be arranged in combination with a movable platform such that a workman may tie his lifeline to the movable platform with an assurance of safety.
It is another object of the invention to provide a safety clamp device which will sense tension in a support or hoist line and automatically engage a safety line when loss of tension is sensed.
It is still another object of the invention to provide a safety clamp device which does not require readjustment of the safety line each time the platform is moved.
It is a further object of the invention to provide a safety clamp device suitable for use with multiple or stacked platforms where men on lower platforms are not aided by lifelines tied to a building or other structure.